In known stretching machines two groups of rollers are provided which are all arranged so that their axes extend in parallel to one another. The two roller groups are customarily arranged at a varying relative vertical spacing and staggered relative to one another. The synthetic film web to be stretched is guided through the roller groups in such a manner that it alternately loops around a portion of the circumference of one roller of one group and of the other group, whereby, each roller is directly driven by means of a separate drive. As a result, the rotational speed of each roller can be selected in a stepless or continuous manner between a minimum value and a maximum value. The lengthwise stretching of the material web takes place between two respective rollers in a stretching gap during operation. In machines for stretching polypropylene and polyester which incorporate the most recent state of the art, the stretching of material is achieved selectively in one or two stretching gaps. For this purpose such lengthwise stretching machines are equipped, for example, with two groups of respectively three stretching rollers each. During the stretching operation, it repeatedly occurs, partly due to varying material qualities, that the film tears. In that event, the film end remaining in the machine becomes rolled up on one of the rollers without an interruption of the roller rotation. In that event a material accumulation occurs, due to the rolling up process, whereby the outer diameter of the affected roller is enlarged. This diameter enlargement then leads to a contacting of the roller pair located directly above or below and, as a consequence, leads to the breaking of the roller shaft ends, damages of the roller bearings, and of the stretching roller itself.
To avoid these shortcomings, suggestions have already been made to use opto-electrical devices for recognizing film tearing and the following undesired wrapping up of the film onto one of the stretching rollers. However, these suggestions have the disadvantage that they lose their functional effectiveness over the course of their operating time due to fouling, or must undergo a substantially constant maintenance to assure the functional effectiveness.
Mechanical means are also known for detecting material accumulations on a roller, such as, for example, in the form of a sliding contact which is constantly in touching contact with the surface of the roller and in the case of a diameter enlargement due to wrapping up of the film, it operates an electrical limit switch which then stops the machine. It is disadvantageous that the wiper contacts constantly are touching the surface of the roller because such constant contact damages the roller surface.